Add snapshot tests for serialised brief payloads

[coderabbitai]

Problem

PR #117 adds unit tests for serialisation functions in episodic/canonical/profile_templates/_brief_serializers.py and related modules, but no snapshot tests exist to validate the complete structured JSON output of those serialisers. Without snapshot coverage, incidental formatting drift or unintended key-set changes in serialised brief payloads can go undetected.

Required resolution

1. Introduce a snapshot testing library (e.g. syrupy) as a development dependency.
2. Add snapshot tests for the serialised outputs of:
   • _serialize_profile_for_brief
   • _serialize_episode_template_for_brief
   • _serialize_reference_document_for_brief
3. Explicitly redact or freeze non-deterministic fields (UUIDs, timestamps) in each snapshot to prevent spurious failures.
4. Add compile-time tests (e.g. TypedDict or assert_type checks) confirming the return types of serialisers are assignable to JsonMapping.

References

• Identified during review of PR Decompose oversized canonical service modules (#92) #117 (module decomposition).
• Raised by @leynos.

/cc @leynos

[coderabbitai]

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